As used herein, the term ‘dynamic range’ (DR) may relate to a capability of the human psychovisual system (HVS) to perceive a range of intensity (e.g., luminance, luma) in an image, e.g., from darkest darks to brightest brights. In this sense, DR relates to a ‘scene-referred’ intensity. DR may also relate to the ability of a display device to adequately or approximately render an intensity range of a particular breadth. In this sense, DR relates to a ‘display-referred’ intensity. Unless a particular sense is explicitly specified to have particular significance at any point in the description herein, it should be inferred that the term may be used in either sense, e.g. interchangeably.
As used herein, the term high dynamic range (HDR) relates to a DR breadth that spans the some 14-15 orders of magnitude of the human visual system (HVS). For example, well adapted humans with essentially normal (e.g., in one or more of a statistical, biometric or opthamological sense) have an intensity range that spans about 15 orders of magnitude. Adapted humans may perceive dim light sources of as few as a mere handful of photons. Yet, these same humans may perceive the near painfully brilliant intensity of the noonday sun in desert, sea or snow (or even glance into the sun, however briefly to prevent damage). This span though is available to ‘adapted’ humans, e.g., those whose HVS has a time period in which to reset and adjust.
In contrast, the DR over which a human may simultaneously perceive an extensive breadth in intensity range may be somewhat truncated, in relation to HDR. As used herein, the terms ‘extended dynamic range’, ‘visual dynamic range’ or ‘variable dynamic range’ (VDR) may individually or interchangeably relate to the DR that is simultaneously perceivable by a HVS. As used herein, VDR may relate to a DR that spans 5-6 orders of magnitude. Thus while perhaps somewhat narrower in relation to true scene referred HDR, VDR nonetheless represents a wide DR breadth. As used herein, the term VDR images or pictures may relate to images or pictures wherein each pixel component is represented by more than 8 bits.
Until fairly recently, displays have had a significantly narrower DR than HDR or VDR. Television (TV) and computer monitor apparatus that use typical cathode ray tube (CRT), liquid crystal display (LCD) with constant fluorescent white back lighting or plasma screen technology may be constrained in their DR rendering capability to approximately three orders of magnitude. Such conventional displays thus typify a low dynamic range (LDR), also referred to as a standard dynamic range (SDR), in relation to VDR and HDR.
As with the scalable video coding and HDTV technologies, extending image DR typically involves a bifurcate approach. For example, scene referred HDR content that is captured with a modern HDR capable camera may be used to generate either a VDR version or an SDR version of the content, which may be displayed on either a VDR display or a conventional SDR display. To conserve bandwidth or for other considerations, one may transmit VDR signals using a layered or hierarchical approach, using an SDR base layer (BL) and an enhancement layer (EL). Legacy decoders that receive the layered bit stream may use only the base layer to reconstruct an SDR picture; however, VDR-compatible decoders can use both the base layer and the enhancement layer to reconstruct a VDR stream.
In such layered VDR coding, images may be represented at different spatial resolutions, bit depths, and color spaces. For example, typical VDR signals are represented using 12 or more bits per color component, while typical SDR signals are represented using 8 bits per color component. Furthermore, base layer and enhancement layer signals may be further compressed using a variety of image and video compression schemes, such as those defined by the ISO/IEC Recommendations of the Motion Pictures Expert Group (MPEG), such as MPEG-1, MPEG-2, MPEG-4, part 2, and H.264.
Layered VDR coding introduces quantization in at least two segments of the coding pipeline: a) during the transformation of the VDR signal from a first bit depth (e.g., 12-bits per color component) to an SDR signal of a second, lower, bit depth (e.g., 8 bits per color component), and b) during the compression process of the base and enhancement layers. False contours may appear on reconstructed images as an artifact of such quantization
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. Similarly, issues identified with respect to one or more approaches should not assume to have been recognized in any prior art on the basis of this section, unless otherwise indicated.